1. Field of the Invention
The invention relates to an optical transmission system and an optical network which are used in a trunk network that covers communication among foreign countries or in the whole country, an intraprefecture network, or the like and, more particularly, to an optical crossconnect apparatus for making a path switching and a bandwidth management of an optical signal.
2. Description of the Related Art
The realization of a large transmission capacity of a communication network is progressing due to a rapid increase in recent data traffic represented by the Internet. Nowadays, since the realization of the optical transmission has been put into practical use, the transmission capacity is increasing by using a time division multiplexing (hereinbelow, abbreviated to TDM) or a wavelength division multiplexing (hereinbelow, abbreviated to WDM). For example, the following transmitting apparatuses have been put into practical use: a transmitting apparatus which can transmit data of 10 gigabits per second per channel; and a wavelength multiplexing transmitting apparatus of a point-to-point type in which channels as many as a few to tens of channels are wavelength multiplexed to one optical fiber by using one fiber and a long-distance transmission exceeding hundreds of kilometers can be possible by using optical amplifiers, regenerators, or the like.
In order to cope with a future increase in demand of the transmission capacity, further economic cost performance, and a variety of services, the following optical networks have been being examined: a ring type optical network in which communication nodes have been connected in a ring shape; and a mesh type optical network in which communication nodes have been connected in a mesh shape in order to increase a degree of freedom of the path route selection. In such optical networks, the following advantages can be expected: the operation is simplified by a network control and management system which supervises each node apparatus in remote-universal manner; what is called an end-to-end path management from a start point to an end point of a circuit is made easy or a path setting speed is raised by the interlocking of monitoring and control units of the node apparatuses. Further, by forming the optical network as a mesh type, a path which needs spare capacity can select a spare capacity in the mesh network, so that there is considered that a spare capacity can be shared by a plurality working paths and the whole network can be economically realized.
To realize the mesh type optical network as mentioned above, there is being developed an apparatus called an optical signal switching apparatus or an optical crossconnect apparatus in which optical signals such as low-speed optical signal like STM-1/OC-3 or the like and high-speed optical signal like STM-64/OC-192, 10GigaBit Ethernet (registered trademark), STM-256/OC-768, or the like can be accommodated as an input/output interface, and a bandwidth management, a path switching, or a switching to a spare apparatus is performed. According to the optical crossconnect apparatus, a connecting relation between the transmission lines connected to a certain node or a connecting relation between the transmission lines and a user apparatus can be changed in an autonomous distributed control manner or a remote and centralized control manner.
In an optical crossconnect apparatus of what is called an O-E-O (optical-electric-optical) type, although the interface between the transmission line out of the apparatus and the user apparatus are optical signals, the efficient signal switching can be realized, since signal switching and line editing processes in the apparatus are realized by an electronic circuit on a unit basis smaller than transmission rate basis, for example, on a unit basis of STS-1 unit which is smaller than that of, for example, STM-64 or OC-192. In an optical crossconnect apparatus of what is called an O-O-O type in which the optical signal is not converted into an electric signal but the switching is performed by using an optical switch, it is expected that information of a large capacity in correspondence to an amount of information which need to be processed in a node which is difficult to be processed by the electronic circuit of the O-E-O type optical crossconnect apparatus can be processed in an O-O-O type optical crossconnect. The O-O-O type optical crossconnect apparatus also includes an optical crossconnect apparatus with a construction in which the optical signal was temporarily converted into the electric signal for a quality monitoring of the signal and a regenerating process at the input or output unit of the optical switch, and it is again processed as an optical signal by the optical switch.
In the case where the number of wavelength in the wavelength multiplexing transmission is increased in order to further increase the transmission capacity, in the node in which fibers from a plurality of lines are accommodated, in order to assure a degree of freedom of connection at the time of connecting the wavelength signals from the lines, it is indispensable to increase the capacity of the optical switch of the optical crossconnect apparatus arranged at the node. As techniques for realizing the optical switch, at present, the following switches have been known: a semiconductor switch or an LiNO3 switch using a refractive index change which is caused by applying an electric field to a material; a PLC (Planar Lightwave Circuit) type switch using a refractive index change which is caused by applying a heat to a material; a movable type optical switch which moves a position of an optical fiber or a lens by using an electromagnet; an MEMS (Micro-Electro-Mechanical Systems) type switch which controls small mirrors formed by a semiconductor technique by using an electrostatic force; and the like.
As MEMS type switches, a 2D type switch and a 3D type switch have been known. According to the 2D type switch, the mirrors are two-dimensionally arranged like a lattice in the vertical and lateral directions and a path of light is switched depending on whether or not the mirror is inserted onto the optical path of an optical signal. Although the mirrors can be simply controlled, since the number of mirrors increases in proportion to the square of the number of input/output ports, it is generally considered to be difficult to construct the optical switch of a scale exceeding 32 inputs and 32 outputs. For example, in order to realize 16 inputs and 16 outputs, 256 (=162) mirror elements are necessary.
According to the 3D type switch, the direction of an optical signal is controlled by continuously changing an angle of a mirror. The 3D type switch is expected as a technique for realizing an optical switch of a large scale exceeding, for example, 32 inputs and 32 outputs. According to the 3D type switch, even in the case of, for example, 32 inputs and 32 outputs, it is sufficient to use the 64 mirror elements in total including 32 mirror elements for the inputs and 32 mirror elements for the outputs. However, according to the 3D type switch, although the number of necessary mirror elements is small, control of the mirrors is complicated more than that of the 2D type switch.
In the case of realizing the optical switch of the large scale by the foregoing optical switch techniques, a method of using one optical switch of a large scale and a method of combining a plurality of optical switches of a small scale are considered. The method of using one optical switch of the large scale has such a problem that an expansion according to the scale of the node cannot be performed. At present, there is a limitation of a scale of the switch which is commercially available from viewpoints of a manufacturing technique and costs. For example, in the 3D type MEMS switch, a scale using up to about (64×64) to (128×128) mirror elements has been realized.
When considering that, at present, there is a limitation in the scale of the optical switch which can be realized by one optical switch and also considering the expandability, it is a practical way to configure the optical switch of the large scale by combining a plurality of optical switches of a small scale. For example, a Clos network in which an optical switch is configured by three stages has been known. As a related art using such an idea, there is JP-A-2002-182250. According to JP-A-2002-182250, a large scale is realized by configuring the optical switch by three stages and, further, a scale of hardware is reduced by using optical circulators. Interfaces which are connected to each side of the optical switch can be connected.
As an example of a technique in which a hardware scale upon expansion is reduced in an apparatus for editing circuit at an electrical level, there is JP-A-2002-77238.